Outlook 550 5.7.1 AS9031519: how to investigate the DMARC bounce

Outlook.com has rejected some incoming messages with 550 5.7.1 and AS9031519, including messages from low-volume sending domains without a DMARC record. This is observed behavior, not an announced Microsoft policy change. Start by checking the sending domain's DMARC record, then investigate SPF and DKIM if the bounce continues.

What does AS9031519 mean?

The reported diagnostic reads:

550 5.7.1 Message rejected: Message lacks SPF/DKIM/DMARC Auth Records (AS9031519)

On November 7, 2026, Spam Resource tested non-bulk domains without DMARC records. Mail to Outlook.com was rejected while mail to a Microsoft 365-hosted corporate domain was accepted. Microsoft Q&A reports also describe low-volume senders encountering this diagnostic. Another Q&A thread describes intermittent rejections.

These observations concern inbound mail to Outlook.com consumer addresses, including Hotmail and Live. They do not establish the same behavior for inbound mail to Microsoft 365 tenants, or prove why every message was rejected.

How to investigate the bounce

1. Read the full non-delivery report. Look for the exact AS9031519 diagnostic in the details. A report can also show an SPF error heading, so check the underlying response.
2. Check the sender's domain. Look up the DMARC TXT record at _dmarc for the domain in the visible From address. The email authentication checker can help inspect the records.
3. If DMARC is missing, publish a record. The domain administrator can add a TXT record at _dmarc with v=DMARC1; p=none, then retry once it is visible in DNS. In the Q&A reports, some senders said the same mail was accepted after adding a record. This is a troubleshooting step, not a guarantee of delivery.
4. Check SPF and DKIM too. Confirm that the sending service is authorized by SPF and signs with DKIM, and that SPF or DKIM aligns with the visible From domain. A DMARC record alone does not fix other authentication failures.
5. If it still fails, ask for help. Send the complete bounce to your email provider or administrator, or contact Outlook sender support. If you are the recipient, share the bounce with the sender: the sending domain's administrator must check its configuration.

Is this the 5,000-per-day rule?

Microsoft's published high-volume sender post covers domains sending more than 5,000 messages per day to its consumer services. The associated rejection is 550 5.7.515, a different diagnostic. Microsoft's sender-support page also describes the high-volume requirements. Neither establishes the reported AS9031519 bounce as an announced policy for all senders.

For the published requirements and their timeline, see Microsoft Outlook.com DMARC enforcement.